Comparing measured and calculated blood loss after a caesarean birth

Introduction Accurate measurement of blood loss after caesarean birth is important for early identification of postpartum haemorrhage. Commonly used visual estimation underestimates blood loss. We compared two methods for objectively assessing blood loss. Methods Measured blood loss was obtained by weighing swabs and pads combined with blood from suction and drapes. Calculated blood loss was derived from maternal weight and peripartum haemoglobin change. Data were from the I'M WOMAN trial investigating tranexamic acid for postpartum haemorrhage prevention. This was a secondary analysis of prospectively collected trial data. We reported median (IQR) blood loss, assessed agreement using Bland-Altman analysis and compared the percentages of women exceeding 500 ml, 1000 ml and 1500 ml. We graphed proportional haemoglobin drop by categories of measured blood loss, stratified by anaemia status. We compared AUCs for measured and calculated blood loss predicting haemodynamic compromise (shock index above 1.0) and death or near-miss. Results A total of 10,393 women were included in this study. Median measured and calculated blood loss were 545 ml (IQR 430-700) and 505 ml (IQR 180-908) respectively, with weak correlation (Spearmans rho=0.28). The IQR was wider for calculated blood loss than for measured blood loss, indicating greater variability. Bland-Altman analysis showed a small mean bias of -39 ml but wide limits of agreement (lower -1196 ml, upper 1118 ml). Measured versus calculated blood loss exceeded 500 ml in 60% versus 51% of women, 1000 ml in 7% versus 21%, and 1500 ml in 2% versus 8%. Women without anaemia had a greater proportional haemoglobin drop than women with anaemia for blood losses below 1000 ml. Measured blood loss had better predictive ability for death or near-miss (AUC 0.87 vs 0.76, p<0.001) and haemodynamic compromise (AUC 0.66 vs 0.62, p=0.001). Conclusion While median measured and calculated blood losses were similar, they were only weakly correlated. Calculated blood loss classified more women as having blood loss above higher thresholds. Women without anaemia had a greater proportional haemoglobin drop than women with anaemia for blood loss below 1000 ml. Measured blood loss had a modest advantage in predicting death or near-miss and haemodynamic compromise, but calculated blood loss remains an objective measure suitable as a trial outcome when direct measurement is not feasible.